An inverse correlation between HDL level and protection against atherosclerosis has emerged from human population studies and the study of transgenic mice. Human HDL is heterogeneous, consisting of two major species, HDL2 and HDL3. In most human population studies, variations in the level of HDL2 have been negatively correlated with atherosclerosis. ApoA-I(Milano) appears to be an exception to the generalization in that modest levels of HDL in these subjects are nevertheless protective, in spite of the fact that HDL3 particles are prominent in these individuals. In this proposal, we will examine the ability of murine and human apoA-Iand apoA-I variants that generate different levels of HDL subclasses to protect against the development of different stages of atherosclerosis (fatty streak lesions and progression to fibroproliferative lesions) and to promote regression of lesions that spontaneously develop in apoE deficient mice. ApoA-I will be introduced into these mice via adenoviral vectors which will allow for the timed expression of apoA-I, as well as manipulation of the amount of protein by varying the dose of virus injected. For long term expression, mice that are deficient in RAG2 as well as apoE are available. Murine and human apoA-I will be compared. Human apoA-I will be expressed in SMC or macrophages (transgenic mice) to assess its impact on atherosclerosis. Other variants will focus on the murine/human chimeric apoA-I and apoA-I(Milano) and related mutants, the latter to assess the importance of heterodimer formation in protecting against atherosclerosis. The influence of human apoA-I and apoA-I variants on atherosclerosis, HDL subclass levels, the ability to stimulate cholesterol efflux and the antioxidant properties of the HDL will be assessed, to attempt correlations among these properties of apoA-I. The Specific Aims are to: 1) determine the relative efficacy of murine and human ApoA-1 in the protection against atherosclerosis in apoA/RAG2 double knockout mice; 2) determine the effectiveness of apoA-1 expressed in the vessel wall in protection against atherosclerosis; 3) determine the relative efficacy of apoA-1 (Milano) (Arg 173-Cys) and other mutants of apoA-1 at position 173 on protection against atherosclerosis; 4) determine the relative efficacy of wild type human apoA-1 and a variant of apoA-1 that produces only HDL2 on protection against atherosclerosis; 5) determine if the high level of expression of apoA-1 has the same protective effect on another model of atherosclerosis; and 6) determine if the protective effect of HDL particles relates to their ability to promote cholesterol efflux or to the antioxidant activity of HDL.